1. Field of the Invention
The present invention is in the field of nuclear magnetic resonance (NMR) signal processing used in exploring earth formations. More specifically, the invention relates to data acquisition methods and pulse sequences that eliminate the effects of ringing in received NMR signals.
2. Description of the Related Art
In typical NMR operations, the spins of nuclei polarize along an externally applied static magnetic field, assumed to be in the z-direction. The vector sum of the magnetic moment from individual nuclei is a macroscopic magnetic dipole called the magnetization, M0. The magnetization is normally aligned with the static magnetic field, but this equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse generated by an RF antenna) which rotates the magnetization away from the static field direction. The length of the RF pulse can be adjusted to achieve a prescribed rotation angle, such as 90°, 180°, etc. After rotating the magnetization away from the static field, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by ω0=γB0, where B0 is the strength of the static field and γ is the gyromagnetic ratio. For hydrogen nuclei, γ/2π=4258 Hz/gauss. So, for example, for a static field of 235 gauss, the frequency of precession would be 1 MHz. Second, the spins return to the equilibrium direction according to a decay time T1, known as the spin-lattice relaxation time. Also associated with the magnetization is a second relaxation called the spin-spin relaxation with a decay time T2.
A widely-used technique for acquiring NMR data both in the laboratory and in well logging uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well-known, after a wait time that precedes each pulse sequence, known as a polarization time, a 90° pulse rotates the magnetization into the x-y plane (transverse plane). This 90° pulse is referred to as a tipping pulse, an excitation pulse or an “A” pulse. Once in the x-y plane, the spins begin to precess around B0 and to lose their relative phase in a process known as dephasing. After a certain time delay, a 180° pulse is applied to cause the spins that are dephasing in the transverse plane to refocus. This 180° pulse is referred to as a flipping pulse, a refocusing pulse or a “B” pulse. Refocusing creates an echo that can be detected by the NMR instrument. By repeated application of the 180° pulse, each application separated by an interecho spacing (TE), a series of “spin echoes” appear, and this series of echoes can be measured and processed. The interecho spacing is typically twice the delay between the 90° pulse and the 180° pulse.
Ringing is a problem encountered when using pulsed NMR techniques. “Ringing” is defined as the presence of a signal appearing in an echo detector window of the NMR tool during the pulsing process, the echo originating from energy stored in some storage means (e.g. electrical or acoustic). In a linear system, the ringing is the sum of signals created by all previous pulses, i.e. by preceding excitation and refocusing (“A” and “B”) pulses.
Techniques for filtering the ringing generated by B-pulses are well known. One may apply a second pulse sequence, where the polarity of the A-pulse is alternated in the second pulse sequence. Alternating the polarity of the A-pulse alternates the polarity of the echo, but does not change the polarity of the B-pulse or the ringing due to the B-pulse. Subtracting the two alternated measurements results in a summation of the echo signals and a subtraction of the ringing signals. This technique is called the “phase-alternate pair” (PAP) technique. The PAP technique is generally accepted in the art as a better technique than those that attempt to measure the ringing directly and then to simply subtract it from the entire signal. Simply subtracting the ringing only makes the signal-to-noise ratio worse. Because the PAP technique effectively averages in another echo, the signal-to-noise ratio is improved by a factor of a √{square root over (2)}. The PAP technique, however, cannot be used to eliminate A-pulse ringing, because the polarity of the A-pulse ringing changes along with the echo.
Another method of eliminating ringing is discussed in U.S. Pat. No. 6,121,774, issued to Sun, et al. Sun '774 advocates measuring the desired echo intensity and the undesired effects during a first pulse sequence. During the single pulse sequence, the spin-echoes, but not the undesired effects, are “spoiled.” After spoiling the spin-echoes during the single pulse sequence, the undesired effects are measured and used to correct the first measured spin-echoes and undesired effects in order to eliminate the ringing component. Sun '774 addresses eliminating ringing due to the 180° pulse, and the method may be used to eliminate ringing from pulses of any length. The method taught by Sun '774 is a “spoiling” method that involves skipping a refocusing pulse.
U.S. Pat. No. 6,466,013, issued to Hawkes, et al., discusses a method of using an optimized rephasing pulse sequence. During the applied sequence, a “B” pulse having a spin tip angle substantially less that 180° is applied with carrier phase shifted by typically π/2 radians with respect to the “A” pulse. Although the refocusing pulses result in spin tip angles of less than 180° throughout the sensitive volume, the RF bandwidth of the “B” pulses is closer to that of the “A” pulse. Hence more of the nuclei originally tipped by the “A” pulse are refocused, resulting in larger echoes than would be obtained with a conventional “B” pulse. The reduced duration of the refocusing pulses also reduces the power consumption of the tool. In one disclosed embodiment, an “A” pulse of inverted phase at the end of the sequence speeds recovery of the longitudinal magnetization by forcing the realignment of the spin system with the static field, enabling cancellation of the tipping pulse “ringing” artifact.
U.S. Pat. No. 6,570,381, issued to Speier, et al., discusses a method of combining a series of cycles of pulse sequences for reducing the effects of ringing. Each of the pulse sequences in Speier '381 includes an RF excitation pulse and several RF refocusing pulses. Spin echo signals are received from the that may include spurious ringing signals from the excitation and refocusing pulses. The spin echo signals are combined from corresponding spin echoes of each of the cycles of pulse sequences to obtain combined spin echo signals in which spurious ringing from the excitation pulses and refocusing pulses of the pulse sequences are substantially cancelled. The steps of producing cycles of pulse sequences and combining spin echo signals include manipulating the polarities of the excitation and refocusing pulses to obtain the substantial cancellation of the spurious ringing from the excitation and refocusing pulses. The disclosure teaches that four cycles of pulse sequences is preferred.
U.S. Pat. No. 6,204,663 to Prammer discusses a method for suppression of magneto-acoustic artifacts in NMR data. In one embodiment at least one first pulse-echo sequence, having a frequency F1, is applied. At least one second pulse-echo sequence, having a frequency F2 which is different from F1, is then applied. The frequency difference is defined as a function of the time delay between excitation pulse and data acquisition, such that:
                                                                  F              1                        -                          F              2                                                =                  1                      (                          4              ⁢              τ                        )                                              (        1        )            where τ is the constant delay time both between the excitation pulse and the first  refocusing pulse (TE/2) and also between the refocusing pulses and the acquisition windows.
In a more general case, the frequency difference can be expressed as:
                                                                  F              1                        -                          F              2                                                =                              (                          n              +                              1                2                                      )                    ⁢                      1                          2              ⁢              τ                                                          (        2        )            in which n is any integer or zero. It will be appreciated that for n=0, Eq. (2) is identical to Eq. (1). The generic Eq. (2) further indicates that a frequency difference corresponding to an additional offset of n/(2τ) will also work due to the cyclic nature of the problem. Since keeping the frequency difference relatively small is desirable, however, the case in which n=0 is preferred. In the Prammer '663 patent only the pulse ringing due to the 90° pulse is removed. A PAP procedure is still needed to remove the ringing from the 180° pulse.